As photolithography in modern semiconductor processing requires higher frequency (shorter wavelength) light in order to create smaller device dimensions within semiconductor devices, harmful effects to the photolithography equipment involved in the processing may result. One such harmful effect occurs when fluoropolymer pellicles are exposed to the high-frequency (high energy) light. Specifically, fluoropolymer pellicles are vulnerable to photochemical darkening when exposed to high-frequency light, which can result in the pellicles having to be replaced more frequently.
One reason for the premature darkening of fluoropolymer pellicles is the destruction of chemical bonds within the fluoropolymer pellicles resulting from the energy transferred from the incident high-frequency light to the pellicles. The darkening caused by the breaking of chemical bonds within the pellicles by the incident high-frequency light reduces the transmission of the light to the underlying semiconductor structure to be exposed to the light.
FIG. 1 illustrates a prior art photolithography system in which a reticle package containing a fluoropolymer pellicle is exposed to high-frequency light transmitted through the mask, then projected unto the wafer. Existing reticle packages typically attempt to reduce the effects of the high-frequency light on the pellicle by producing pellicles that are more transparent and therefore less likely to react with the incident light. Because it is difficult to produce a pellicle that is truly transparent, some photons from the incident light are still absorbed, resulting in a degradation of the pellicle's transparency and lifespan.
FIG. 1 further illustrates the use of N2 purging in order to dispel photon-absorbing compounds, such as O2 and H2O found in the atmosphere surrounding the pellicle.
Exposure of pellicles to a combination of O2 and N2 has been shown to decrease the destructive effects of high-frequency incident radiation without substantially attenuating the intensity of the radiation when used in proper amounts. Furthermore, other purge gas mixtures, such as H2/N2, F2/N2, and F2/H2/N2, as well as fluorocarbon gases, such as CF4, and C2F6, or a mixture of O2 with these gases can be used as a suitable purge gas to help extend the transparency life of pellicles. Other fluorocarbon (FC) gases or hydrofluorocarbon (HFC) gases may also be used as purging gases.
Excessive amounts of these purge gases, however, can attenuate the intensity of an incident radiation, thereby altering the intended effect upon device features of the semiconductor. For example, exposing an entire reticle to these purge gases could decrease the destructive effects by the incident radiation to the pellicle of FIG. 1, but would attenuate the incident radiation intensity such that the light would not properly react with the exposed silicon features on the wafer.